The invention relates to a circuit arrangement for generating a sawtooth shaped voltage across a capacitor. The invention provides charging current means for charging said capacitor until a first threshold voltage is reached and a discharging current means subsequently discharging the capacitor until a second threshold voltage is reached. Additionally means are provided for measuring and controlling the capacitor voltage and for switching said charging and discharging current means, respectively, the free-running frequency of the sawtooth voltage thus generated being determined by the difference between said first and second threshold voltages at constant charging and discharging currents or alternatively said frequency is determined by the intensity of said charging and discharging currents, respectively, at a constant difference between said first and second threshold voltages.
Such a circuit arrangement, which is known from British Patent No. 1,245,552, has the advantage that, except for the capacitor, it does not comprise a frequency-determining element so that integration thereof in a semiconductor body is possible, which furnishes a standard circuit usable for different frequencies and which requires few input and output terminals. It is, for example, possible to use circuits which are identical but for the capacitor, for generating, in a television receiver, a sawtooth voltage at field frequency for the vertical deflection, a sawtooth voltage at line frequency for the horizontal deflection, and a sawtooth voltage for controlling the switch in a switched-mode power supply, the frequencies of the generated voltages being, for example 50 Hz, 15,625 Hz (European standard) and 18 kHz, respectively. These three sawtooth oscillators can be advantageously integrated in one and the same semiconductor body.
Setting the frequency of the generated sawtooth is rather simple in the known circuit: at a constant charging and discharging current the difference between the first and the second threshold voltage is set to a certain value. This results in a certain amplitude of the sawtooth and it will be clear that this determines the frequency. It is also clear that the frequency can be controlled by a control of this amplitude. Such a control can be used in a loop for an indirect synchronization circuit wherein a phase discriminator converts a phase difference between the sawtooth generated by the oscillator and incoming synchronizing pulses into a control voltage which influences the amplitude of the sawtooth in such a manner that said phase difference becomes smaller. This can, for example, be effected because one of the threshold levels is shiftable under the influence of the control voltage whereas the other threshold level remains constant. At a constant amplitude of the sawtooth the charging or discharging current, can be controlled in a similar manner.
A frequency control as outlined in the preceding paragraph always has a certain inertia and always causes a small phase difference which cannot be fully eliminated. Furthermore, a control is hard to realize for comparatively low frequencies. For a field frequency of, for example, 50 Hz the control voltage should remain constant for 20 ms.
It is an object of the invention to provide a sawtooth oscillator of the type defined above, whose frequency can be influenced by direct synchronization and which, consequently, need not satisfy critical requirements. The invention is based on the recognition to choose an oscillator which, in the absence of synchronizing pulses, has a free-running (natural) frequency which is substantially equal to the nominal repetition frequency of said pulses. Nominal frequency is here understood to mean the frequency which the pulses should have, for example in accordance with a standard, although in practice they can deviate somewhat therefrom owing to tolerances. This approach has the advantage that the oscillator can be set in a simple manner without a synchronizing signal as the frequency thereof is the same as the frequency in the synchronized state with a synchronizing signal. Furthermore, should the synchronizing signal disappear temporarily for some reason the oscillator continues to keep the nominal frequency of this signal.
If, however, the natural frequency of the oscillator varies in the absence of a synchronization signal, for example owing to tolerances, ageing of components or due to temperature effects, in such a manner that the natural frequency assumes a value which is higher than the nominal value, then the oscillator would not be synchronizable without further measures, as now the period of the sawtooth is shorter than the time interval between two consecutive synchronizing pulses.